Noninvasive determination of exercise-induced vasodilation during bicycle exercise using near infrared spectroscopy

The purpose of this study was to examine the changes in total hemoglobin (Δ[tHb]) response during bicycle exercise at various constant workloads using near infrared continuous wave spectroscopy (NIRcws) in humans. We hypothesized that the Δ[tHb] during exercise may progressively increase as a result of a dilation of the vascular bed and/or capillary recruitment at lower constant work rates. Seven healthy subjects performed bicycle exercise at 20, 40, 60, 80, and 100% of maximal work rates (Wmax) for 5 min. Muscle oxygenation change (Δ[Oxy]) and Δ[tHb] at the right vastus lateralis were monitored using a NIRcws device. Exercise-induced Δ[tHb] and Δ[Oxy] responses at each constant workload were evaluated as functional Δ[tHb] change (f-Δ[tHb]) and functional oxygenation change (f-Δ[Oxy]), respectively. Blood lactate concentration [La] was also evaluated after each exercise stage. At work rates 60%Wmax and below, after an initial decrease at the start of exercise, both Δ[tHb] and Δ[Oxy] showed progressive increases until the end of exercise. A significant positive correlation was found between f-Δ[tHb] and f-Δ[Oxy] (p<0.01). In addition, there was a significant negative relationship of [La] to f-Δ[tHb] during exercise (p<0.05). These results provide evidence that increased muscle oxygenation during bicycle exercise up to 60%Wmax may be caused by increased O 2 supply due to exercise-induced blood volume expansion. Subsequently, the cessation of increase in f-Δ[tHb] at higher intensity exercise may lead to lower muscle tissue oxygenation and higher lactate accumulation.